When field devices are to be connected to a bus line in areas at risk of explosion, explosion protection measures have to be taken. In the case of solutions known in practice, controlled or regulated bus voltage sources are used by way of example, which are connected to the bus line in order to supply energy to the entire data bus inclusive of the field device coupler and the field devices. Depending on the used field devices, the supply voltage across the bus line must be limited here to 17.5 V (field devices according to FISCO) or to 24 V (field device according to ENTITY classification). The permissible line lengths of the bus line are thus severely limited in turn.
In order to obtain inherently safe electrical circuits for an area at risk of explosion, DE 199 10 409 A1 suggests connecting a field bus distributor to the field device coupler on the output side. This field bus distributor has means for current limitation and means for voltage limitation, such that inherently safe connections are achieved there for field devices. Resistors are used for current limitation. The voltage limitation is effected via Zener diodes. A short-circuit current, which trips the fuses, is created via the Zener diodes by overvoltages.
Another solution for areas at risk of explosion is known from EP 1 965 482 B1. The bus line is divided there into galvanically isolated bus line portions supplied separately from one another. A signal transmission with direct components between the bus line portions is prevented by the galvanic isolation. Communication signals having an alternating component are transmitted between the bus line portions. This solution, however, is complex and costly. It requires a large number of energy supply units for each bus line termination and a galvanic isolation means (for example transmitter) between each two bus line portions.